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SECTION V
Gastrointestinal Physiology
DIGESTION & ABSORPTION:
CARBOHYDRATES
DIGESTION
The principal dietary carbohydrates are polysaccharides, dis-
accharides, and monosaccharides. Starches (glucose poly-
mers) and their derivatives are the only polysaccharides that
are digested to any degree in the human gastrointestinal tract.
Amylopectin, which constitutes 80–90% of dietary starch, is a
branched molecule, whereas amylose is a straight chain with
only 1:4
α
linkages (Figure 27-1). The disaccharides
lactose
(milk sugar) and
sucrose
(table sugar) are also ingested, along
with the monosaccharides fructose and glucose.
In the mouth, starch is attacked by salivary
α
-amylase.
However, the optimal pH for this enzyme is 6.7, and its action
is inhibited by the acidic gastric juice when food enters the
stomach. In the small intestine, both the salivary and the pan-
creatic
α
-amylase also act on the ingested polysaccharides.
Both the salivary and the pancreatic
α
-amylases hydrolyze
1:4
α
linkages but spare 1:6
α
linkages and terminal 1:4
α
link-
ages. Consequently, the end products of
α
-amylase digestion
are oligosaccharides: the disaccharide
maltose;
the trisaccha-
ride
maltotriose;
and
α
-limit dextrins,
polymers of glucose
containing an average of about eight glucose molecules with
1:6
α
linkages (Figure 27–1).
The oligosaccharidases responsible for the further digestion
of the starch derivatives are located in the brush border of small
intestinal epithelial cells (Figure 27–1). Some of these enzymes
have more than one substrate.
Isomaltase
is mainly responsible
for hydrolysis of 1:6
α
linkages. Along with
maltase and
sucrase,
it also breaks down maltotriose and maltose. Sucrase
and isomaltase are initially synthesized as a single glycoprotein
chain which is inserted into the brush border membrane. It is
then hydrolyzed by pancreatic proteases into sucrase and iso-
maltase subunits.
Sucrase hydrolyzes sucrose into a molecule of glucose and a
molecule of fructose. In addition, two disaccharidases are
present in the brush border:
lactase,
which hydrolyzes lactose to
glucose and galactose, and
trehalase,
which hydrolyzes treha-
lose, a 1:1
α
-linked dimer of glucose, into two glucose molecules.
Deficiency of one or more of the brush border oligosaccha-
ridases may cause diarrhea, bloating, and flatulence after
ingestion of sugar (Clinical Box 27–1). The diarrhea is due to
the increased number of osmotically active oligosaccharide
molecules that remain in the intestinal lumen, causing the
volume of the intestinal contents to increase. In the colon,TABLE 27–1
Normal transport of substances by the intestine and location of maximum absorption or secretion.
a
Small IntestineAbsorption of: Upper
b
Mid Lower Colon
Sugars (glucose, galactose, etc) ++ +++ ++ 0
Amino acids ++ ++ ++ 0
Water-soluble and fat-soluble vitamins except vitamin B
12
+++ ++ 0 0
Betaine, dimethylglycine, sarcosine + ++ ++?
Antibodies in newborns + ++ +++?
Pyrimidines (thymine and uracil) + +??
Long-chain fatty acid absorption and conversion to triglyceride +++ ++ + 0
Bile acids + + +++
Vitamin B
12
0 + +++ 0
Na
+
+++ ++ +++ +++
K
+
+ + + Sec
Ca
2+
+++ ++ +?
Fe
2+
+++ + +?
Cl- +++ ++ + +
SO
4
2–
++ + 0?
a
Amount of absorption is graded + to +++. Sec, secreted when luminal K
- is low.
b
Upper small intestine refers primarily to jejunum, although the duodenum is similar in most cases studied (with the notable exception that the duodenum secretes HCO
3 –
and shows little net absorption or secretion of NaCl).